Emulsions constitute an important product type in the field of cosmetic, dermatological and/or pharmaceutical formulations. Cosmetic formulations are utilized essentially for skincare. Skincare, in the cosmetic sense, is primarily the enhancement and/or reestablishment of the natural function of the skin as a barrier against environmental influences (for example, soil, chemicals, microorganisms, etc.) and against the loss of endogenous substances (for example, water, natural fats, electrolytes, etc.).
A further aim of skincare is to compensate for the loss of fats and water in the skin caused by daily washing and to preserve and restore the softness and smoothness of the skin. This is important when the natural regeneration capacity is insufficient. Moreover, skincare products should provide protection from environmental influences, especially from the sun and the wind, and delay skin ageing.
Pharmaceutical topical compositions generally comprise one or more medicaments in an effective concentration. For the sake of simplicity, cosmetic and medical use and corresponding products are clearly distinguished by reference to the legal stipulations of the Federal Republic of Germany (for example, Cosmetics Act, Food and Drug Laws).
In the last few years, cosmetic wet wipes have gained increasing significance owing to their extremely simple and convenient usability. Initially, virtually exclusively wet wipes for cleaning purposes were represented on the cosmetics market, which comprised mainly aqueous, surfactant-containing impregnating solutions. However, in recent times, care products have also been appearing more and more on the market, which are based on impregnating emulsions and thus additionally comprise a care oil component.
Most of these cosmetic wet wipes for bodycare and facecare are impregnated with emulsions which have been prepared by the PIT emulsifying method (see, for example, K. Shinoda, H. Kunieda, Phase properties of emulsions: PIT and HLB, Encycl. of Emulsion Technology, 337-367 (1), 1983 or Th. Förster, F. Schambil, W. von Rybinski, J. Disp. Sci. And Technology, 13(2), 183-93 (1992)).
The PIT method makes use of the fact that, in an oil-in-water (O/W) emulsion which is stabilized by nonionic emulsifiers containing polyethylene glycol (“PEG-containing emulsifiers”), a phase inversion can be induced to give a water-in-oil (W/O) emulsion by increasing the temperature (phase inversion; PIT: phase inversion temperature).
Since the water-oil interface tension is extremely low in this phase inversion region, extremely fine oil-in-water emulsions can thus be obtained after cooling. For this purpose, it is, however, necessary that the individual components of the emulsions are adjusted precisely with respect to one another for each system to be emulsified. This means that emulsifier mixtures and emulsifier concentration have to be “tailored” for different oil phases.
The fine and low-viscosity emulsions thus produced have excellent long-term stability and are thus very suitable as impregnating solutions for wet wipes. Such systems are described, for example, in EP-B-1 268 740 or WO-A-00/04230.
A fundamental requirement for the use of PIT emulsification technology is, as described, the necessity for the entire emulsion to be heated to temperatures above the phase inversion temperature and thereafter to be cooled down.
At the present time, where process operations have to be optimized and energy costs restricted, this means a distinct disadvantage compared to systems which do not have to pass through this heating/cooling curve. For this reason, fine, prolonged-stability emulsions which can be prepared at room temperature (“cold preparation”) without having to pass through an additional heating/cooling curve would be advantageous.
Another disadvantage in impregnating solutions for wet wipes based on PIT emulsions is that such wet wipes are based on the use of PEG-containing emulsifiers. In view of very natural cosmetic formulations, it is an important aim of cosmetic research to be able to dispense with emulsifiers containing polyethylene glycol (“PEG”). There is therefore an increased search for PEG-free alternative solutions.
It is also known that ethoxylated emulsifiers impart a rather watery skin feel, which can be improved sensorily by the use of, for example, polyglyceryl esters.
For instance, WO-A-02/056841 describes PEG-free impregnating emulsions for cosmetic wet wipes based on polyol poly-12-hydroxystearates and alkyl glycosides. The use of these emulsifier mixtures leads to improved softness of paper products impregnated with them and also leads to improved sensory properties in use of the wet wipes produced with them. In the case of such emulsifier combinations, it is, however, generally difficult to achieve good long-term stability of the impregnating emulsions in combination with sufficient preservation.
Especially in the production of wet wipes, sufficient preservation of the impregnating solutions is absolutely necessary in order to prevent germ growth. The preservation has to be sufficient to protect both the impregnating solutions themselves and finally also the impregnated wet wipes in the long term against germ growth.
Preferred preservative mixtures used are typically mixtures of alkylparaben esters and phenoxyethanol, as are commercially available, for instance, under the trade names Euxyl® K 300 (Schülke & Mayr) or Phenonip® (Clariant).
The described requirements regarding reliable preservation of impregnating solution and wet wipes make it necessary that relatively large amounts of these alkylparaben ester/phenoxyethanol mixtures generally have to be used in the finished impregnating solutions (0.5 to 1.0% by weight). Ideally, the entire amount of preservative should be incorporated as early as in the production of emulsion concentrates as possible. This allows the desired use concentration of the impregnating solution to be established subsequently in a simple manner by dilution with water.
It is known that the use of these alkylparaben ester/phenoxyethanol mixtures has an emulsion-stressing influence, since these compounds are very interface-active and compete with emulsifier molecules for a space at the oil-water interface. Owing to the interface-active character of these preservative mixtures, they can therefore also be described as aromatic cosurfactants with preservative properties. In the case of impregnating emulsions for wet wipes, this emulsion-stressing effect is generally enhanced by the required high amounts of these preservatives and the low viscosities of the impregnating solutions.
In summary, it can therefore be stated that it is not possible with the emulsifiers or emulsifier combinations described in the prior art to prepare cold-preparable, sufficiently preserved, low-viscosity, fine and prolonged-stability emulsions, as are typically used for impregnating emulsions or sprayable lotions.